Context : Low and intermediate mass stars are known to power strong stellar winds when evolving through the asymptotic giant branch ( AGB ) phase .
Initial mass , luminosity , temperature , and composition determine the pulsation characteristics of the star and the dust species formed in the pulsating photospheric layers .
Radiation pressure on these grains triggers the onset of a stellar wind .
However , as of today , we still can not predict the wind mass-loss rates and wind velocities from first principles neither do we know which species are the first to condense in the upper atmospheric regions .

Aims : We aim to characterise the dominant physical , dynamical , and chemical processes in the inner wind region of two archetypical oxygen-rich ( C/O < 1 ) AGB stars , that is , the low mass-loss rate AGB star R Dor ( \dot { M } \sim 1 \times 10 ^ { -7 } M _ { \odot } /yr ) and the high mass-loss rate AGB star IK Tau ( \dot { M } \sim 5 \times 10 ^ { -6 } M _ { \odot } /yr ) .
The purpose of this study is to observe the key molecular species contributing to the formation of dust grains and to cross-link the observed line brightnesses of several species to the global and local properties of the star and its wind .

Methods : A spectral line and imaging survey of IK Tau and R Dor was made with ALMA between 335 and 362 GHz ( band 7 ) at a spatial resolution of \sim 150 mas , which corresponds to the locus of the main dust formation region of both targets .

Results : Some two hundred spectral features from 15 molecules ( and their isotopologues ) were observed , including rotational lines in both the ground and vibrationally excited states ( up to v=5 for SiO ) .
Detected species include the gaseous precursors of dust grains such as SiO , AlO , AlOH , TiO , and TiO _ { 2 } .
We present a spectral atlas for both stars and the parameters of all detected spectral features .
A clear dichotomy for the sulphur chemistry is seen : while CS , SiS , SO , and SO _ { 2 } are abundantly present in IK Tau , only SO and SO _ { 2 } are detected in R Dor .
Also other species such as NaCl , NS , AlO , and AlOH display a completely different behaviour .
From some selected species , the minor isotopologues can be used to assess the isotopic ratios .
The channel maps of many species prove that both large and small-scale inhomogeneities persist in the inner wind of both stars in the form of blobs , arcs , and/or a disk .
The high sensitivity of ALMA allows us to spot the impact of these correlated density structures in the spectral line profiles .
The spectral lines often display a half width at zero intensity much larger than expected from the terminal velocity , v _ { \infty } , previously derived for both objects ( 36 km/s versus v _ { \infty } \sim 17.7 km/s for IK Tau and 23 km/s versus v _ { \infty } \sim 5.5 km/s for R Dor ) .
Both a more complex 3D morphology and a more forceful wind acceleration of the ( underlying ) isotropic wind can explain this trend .
The formation of fractal grains in the region beyond \sim 400 mas can potentially account for the latter scenario .
From the continuum map , we deduce a dust mass of \sim 3.7 \times 10 ^ { -7 } M _ { \odot } and \sim 2 \times 10 ^ { -8 } M _ { \odot } for IK Tau and R Dor , respectively .

Conclusions : The observations presented here provide important constraints on the properties of these two oxygen-dominated AGB stellar winds .
In particular , the ALMA data prove that both the dynamical and chemical properties are vastly different for this high mass-loss rate ( IK Tau ) and low mass-loss rate ( R Dor ) star .